1. Field of the Invention
This invention relates to an exhaust gas purifying catalyst and a method for the purification of exhaust gas. More particularly, it relates to a method for catalytically reducing harmful substances, particularly nitrogen oxides (NOx), contained in the exhaust gas with a hydrocarbon as a reducing agent, an exhaust gas purifying catalyst capable of adsorbing simultaneously the hydrocarbon and NOx in the exhaust gas thereby decomposing and reducing NOx in a wide temperature range without introducing any hydrocarbon by way of replenishment from the external source, and a method for the purification of the exhaust gas by the use of the catalyst.
2. Description of Related Art
The NOx in the atmospheric air causes photochemical smog and acid precipitation. Thus, the emission of NOx from mobile generative sources such as automobiles that are furnished with internal combustion engines such as gasoline and diesel engines, which form one of the generative sources for NOx, has become one of the serious social issues. Official studies are now under way with a view to rigidifying legal regulations on the tolerance of NOx emission in the future. In the light of the demand of such social cause, efforts are being promoted with a view to developing an exhaust gas purifying catalyst.
As the catalyst capable of purifying the exhaust gas of a gasoline engine, the so-called three-way catalyst, i.e. a catalyst capable simultaneously reducing NOx, unburnt hydrocarbon, and carbon monoxide, has been heretofore used. The three-way catalyst, when the ordinary gasoline engine is used and the exhaust gas therefrom contains substantially no oxygen, is capable of efficiently reducing NOx with the unburnt hydrocarbon and the carbon monoxide in the exhaust gas.
The exhaust gas from the diesel engine contains oxygen in great excess on account of the special quality of the engine, and theoretically has small contents of hydrocarbon and carbon monoxide as a reducing agent for NOx. When the ordinary three-way catalyst is used, it is nearly incapable of reducing NOx for the treatment of exhaust gas from the diesel engine.
In recent years, gasoline engines of type of a lean burn and an in-cylinder fuel injection system have been developed. Since these engines operate by lean burn of the fuel, their exhaust gases contain oxygen in high concentrations. When the ordinary three-way catalyst is used, it purifies NOx with difficulty for the treatment of such an exhaust gas of engines.
JP-A-63-100919 proposes a catalyst having copper deposited on such a porous carrier as zeolite and JP-A-5-137963 proposes a method using platinum as a main catalyst as respects the exhaust gas purifying catalyst effective in the removal NOx in the exhaust gas containing oxygen copiously such as the exhaust gas of a diesel engine or a lean burn gasoline engine.
These methods heretofore proposed have, as conditions for attaining efficient decomposition of NOx, the point that at least the temperature is limited in a comparatively narrow range and the point that the amount of the hydrocarbon as a reducing agent is comparatively large. These conditions are not easily applied to the actual exhaust gas that varies with the kind of engine and the conditions of travel.
Not only the copper and platinum type catalysts mentioned above but also the catalyst in general is incapable of purifying NOx unless the catalyst bed is heated to a certain degree by the exhaust gas from the engine. In the diesel engine whose exhaust gas has a comparatively low temperature, it is extremely difficult for the conventional technique to purify NOx that is generated from immediately after the start of the engine through the duration of idling and low-speed travel.
From this view, a NOx adsorbent that is not appreciably affected by the exhaust gas temperature and the gas composition as compared with NOx purifying catalyst has been proposed. Specifically, JP-A-9-57093 discloses an adsorbent having nickel oxide and copper oxide as main components and aimed at the NOx contained in the atmospheric air or in various kinds of roadside exhaust gas, and JP-A-7-47227 discloses an adsorbent having iron oxide hydrate as a main component and aimed particularly at NOx contained in the exhaust gas in an automobile tunnel or an indoor parking lot. The NOx adsorbents thus proposed, however, have limited capacities for adsorption, are incapable of being continuously used and needed regeneration as an indispensable condition. Further, these NOx adsorbents are generally used under conditions such as a fixed temperature and a gas composition variable only sparingly. Even when NOx adsorbent is used on the exhaust gas from a diesel engine, since the exhaust gas is always in a lean atmosphere, the desorption of NOx by the reducing gas and the regeneration of the reducing agent by purification cannot be hoped for. Further, since NOx adsorbent has the behavior of adsorption and desorption of NOx largely depending on temperature, it entails the problem that NOx adsorbed in a low temperature range, on exposure to a high temperature, is inevitably desorbed without being purified.